Method of precipitating solid particles from a suspension of the particles in a liquor



Dec. 29, 1953 A. SABLE I METHOD OF PRECIPITA'IING sown PARTICLES FROM ASUSPENSION OF THE PARTICLES IN A LIQUOR 2 Sheets-Sheet 1 Filed July 251949 INVENTOR.

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Dec, 29. 1953 A SUSPENSION OF THE PARTICLES IN A LIQUOR Filed July 25,1949 2 Sheets-Sheet 2 ZNVENTOR. I flzvaxgs 64121 E,

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Patented Dec. 29, 1953 METHOD OF PRECIPITATING SOLID PAR-i TICLES FROM ASUSPENSION OF THE PARTICLES IN A LIQUOR Andr Sabl, Marseille, France,assignor to Societe dElectro-Chimie, dElectro-Metallurgie et desAcieries Electriques dUgine, Paris, ,France, a corporation of FranceApplication July 25, 1949, Serial No. 106,653 Claims priority,application France August 7, 1948 The present invention relates to amethod for insuring the maintenance of solids in suspension in a liquidand which furthermore permits a continuous circulation of the liquidphase independently of the movements of the solids. An apparatus forcarrying out the method is illustrated herein, and is claimed in adivisional application Serial No. 206,327, filed January 1'7, 1951.

In order to efiect the putting and the maintenance in suspension of asolid in a liquid it is possible to usemechanical stirring devices themovement of which creates eddies which cause a more or less homogeneousdistribution of the solid to be maintained in suspension in the liquid.

It is also possible to admit to the bottom of the suspension vessel arising current of compressed gas which also creates eddies. This methodofiers the advantage with respect to the preceding method, that contactwith the mass to be stirred, by any mechanical member, is avoided.

In spite of their simplicity both these first methods are not veryeconomical from the point of view of the consumption of power, since themajor part of the energy which is used is dissipated infriction betweenthe liquid veins.

It is also possible to create in the suspension vessel a rising currentof liquid the speed of which is equal to, and in the opposite directionto the speed of the falling grains of the solid.

-But this third method though more rational can be applied economicallyonly with grains which are rather closely calibrated. As a matter offact, the grains which are generally of an unequal size fall in theliquid at unequal speeds, the coarser grains falling more quickly. Inorder to maintain a rising current of liquid the speed of which is equalto that at which the grains fall, and opposite in direction, it isnecessary, therefore, that said current have different speeds adapted toevery grain size. This may be obtained by using a conical vessel.Indeed, the speeds of a liquid at the different levels of such a vesselare proportional to the cross-sections of said vessel, and rising speedsare thus obtained for the liquid which are in opposition to the fallingspeeds of the various grains. However, the vertex angle of the conicalvessel must remain small (about 7) since otherwise excessive eddieswould be produced. Accordingly, the speeds of the liquid are comprisedwithin a narrow range, 4

corresponding, in the ".same manner, to only a. narrow range'incalibration of the grain size.

Lastly, it is possible to spread on the surface of the liquid the solidparticles to be maintained in suspension therein, to allow saidparticles. to

4 Claims. (Cl. 23-301) fall freely through the whole height of theliquid, and then to collect them in the lower part in a state of asuspension which has been thickened or concentrated, and to raise themagain to the surface by any mechanical method, for example by means of apump.

This fourth method offers the following advantages with respect'to thepreceding methods:

(a) The power used in any case is proportional to the mean falling speedof the particles to be maintained in suspension, and no longer to thefalling speed of the largest grains which is higher.

(b) It is possible to maintain in suspension a solid of anygranulometric composition without any delicate adjustment. 7

(c) The available volume is better utilized and the suspension is morehomogeneous. Indeed, the bad distribution of the grains which is causedby the other methods is avoided.

However, this fourth method still generally requires the'use of 'amechanical device (pump). Furthermore, it does not do away with theliquidmetal friction at a'high speed.

The problems of putting and maintaining in suspension are particularlydifilcult of solution in a simple and economical manner for devices oflarge dimensions, more particularly in the case of dense grains havingrather large dimensions (for example from a few tenths of a to a fewmillimeters) and capable of decanting in a compact layer which is notvery plastic, or not at all plastic (the grains coming into contact withone another and being no longer carried by the liquid phase).

In this state of sedimentation (for example resulting from an accidentalstoppage of the power) it may happen that resuspension becomesimpossible either because of impaction or an insuflicient power of themechanical elements or because the liquid eddies are ineffective todisintegrate the sedimented mass.

The present invention has for its object to provide a method and adevice which make it possible to remedy these disadvantages and,furthermore, as will be seen later on, to obtain independence of themovements of the solids and the liquid.

The method in accordance with the invention essentially consists ininsuring the suspension of the solid phase in the liquid in a vesselcontaining the liquid, by means of an injection of gas into the lowerpart of this vessel; in causing the so obtained suspension to rise in acontinuous manner through a central tube, preferably a telescopic tube,the lower part of which can occupy an adjustable position with respectto the bottom of the vessel, up to the upper part of the device; and toallow a continuous decantation of the solid in said upper part by meansof a device which comprises one or more decantation rings whichcommunicate with the vessel in three superposed planes, the suspensionentering the ring or rings through the median plane, the heavy particlesreturning to thecircuit through the lower plane, the light particles orparticles carrying air bubbles returning to the suspension through theupper plane which is located above the normal level of the liquid in theenclosed space, and the clear liquid being discharged through anoverflow located substantially at the same height as the upper plane.

The device for carrying outthe above described method fundamentallycomprises, therefore, a vessel, preferably conical at its lower part,which is connected at said lower part to a source of compressed gas.Inside said vessel a preferably telescopic tube is vertically disposedin alignment with the gas supply, the lower opening of said tube beingadjustable with respect to the bottom of the vessel. Its upper openingis provided with means for disengaging the emulsion and distributing thethick suspension over the Whole surface of the vessel. The upper part ofthe vessel is surrounded by one or more preferably frusto-conical ringswhich communicate with the said vessel through a series of openingsprovided in three different horizontal planes, the upper plane lyingabove the normal level of the liquid in the vessel. The rings may beprovided, on the one hand, with an overflow lying substantially at thesame height as the above mentioned upper plane and, on the other hand,with a cylindrical ferrule the base of which lies slightly below thehorizontal discharge level.

The method in accordance with the invention combines the advantages ofsimplicity of the second. above mentioned method (absence of mechanicalelements) with those of the fourth method (conservation of power,flexibility of use, homogeneousness of the suspension).

The conservation of power, more particularly, is enhanced to its maximumby the fact that the friction surfaces are reduced to a strict minimumby the substitution for friction between an. incompressible liquid and awall, of friction between a resilient emulsion and a wall, and to theelimination of friction at a high speedwhich consumes energy and givesrise to wear. This result is obtained by using gas compressed to apressure which is veryslightly higher than the hydrostatic pressurewhich opposes its outlet from the injection nozzles.

The device which is used is effectively an industrial device, forworking incidents (unforeseen stoppages, clogging of the walls, casualpresence of foreign bodies) entail only a minimum of inconveniences. Itthus provides a practical and economical solution to the problems of thesuspension of solids with a true sedimentation. The actuation of themovable lower part of the telescopio tube provides a powerful means forthe dis ntegration of the sedimented mass to be brought into suspension.

Furthermore, the method maybe adapted to a continuous operation even if,more. particularly, the movements of feeding and discharging of theliquids and solids in suspension are such that they have to be effectedindependently of one another, or if the mean sojourn times for theliquid and the solid must be different, or if in complicated cases, bymounting in series as many devices as necessary and by providing, in

each of said devices, for the independence of the supply and outletmovements of the liquid and of the solids.

This result is obtained by associating with each vessel containing thesuspension, a special continuous settling device such as the devicewhich will be described later on, and which fixes in position a smallpart of the volume in the form of a liquid which is more or less freefrom suspension, and makes possible not only the separation of thenormal dense grains (returning through the lower part) but also that ofthe grains to whichair bubbles adhere (returning through the upperpart).

. Lastly, the method in accordance with the invention makes possible, inthe course of the operation with a continuous flow, a lexibleandprogressive granulometric sorting out of the grains or crystals insuspension.

The adaptability to progressive selection results:

(1) from the mode of circulation adopted for maintaining the solid insuspension, which makes possible a concentration, in the rising column,of the grains falling with the highest speed, and which thus may bedischarged-in a preferential manner by taking them off at any point oftheir travel; The so obtainedselection is progressive if it is repeatedwith a plurality of devices inseries.

(2) from the operation of the decanter which makes possible, for a givenoutput, the carrying along of the solid particles falling with a speedwhich is lower than a predetermined limit, by the lye which flowsthrough said decanter and which is discharged into the succeedingdevice.

The method in accordance with the invention may be used for all sorts ofapplications, more particularly when slow reactions are involved (thus,reactions requiring the carrying out of large masses) which may bephysical, chemical or simply mechanical reactions between liquids andsolids with or without interventionof a gas (which, when used in a moreor less closed circuit, may act both as a reagent and as the drivingfluid).

It-may be used for the settling,- sorting out, exhausting, ordisintegration of solids, or even, because of its small consumption ofenergy, for the stocking and destocking of products which must or may bemaintained in a wetted condition.

The possibilities of adjustment which it com prises (position of themovable part of the telescopic tube) make it possible to vary at willthe relative richness of the product in suspension in the rising column(from whence it is possible to effect the extraction) with respect tothat of the remainder of the vessel.

It is possible, for example, to decant with a constant richness(solid/liquid) in spite or vari-' ations in the total quantity of solidin suspension in the whole of the device.

The invention may be applied in a particularly interesting manner to thedecomposition, in a continuous operation, of sodium aluminate lyes inthe method of Bayer. The description thereof, which is given hereunder,by way of nonlimitative example, will contribute to a betterunderstanding of the object and of the form of execution of theinvention.

It is known that the use of continuous devices for the decomposition ofthe sodium aluminate lyes of Bayer offers numerous advantages over thediscontinuous treatment, such as elimination of dead times, control ofthe granulometry, saving in the volume of lye which is fixed inposition, and of the sheet metal surface of the precipitators.

It is also known that in order to obtain both high volumetriceiliciencies and a good exhausting of the lye, the operation must beconducted successively in a plurality of precipitators mounted inseries, and in which the sodium aluminate lye is decomposed throughhydrolysis with a crystallization of alumina hydrate on already formednuclei.

The precipitation of Bayer has the characteristic that in order toreconcile high volumetric efliciencies with the production of welldeveloped hydrate crystals it is necessary to maintain in suspension animportant quantity of these already formed nuclei (at least 200 gramsper litre).

Now, the aluminate lye gives up during its passage throu h all of theprecipitators only '70 to 85 grams of hydrate per litre (in three to sixdays). Moreover, this quantity is fractionated in each of the devices,the most favoured of which (that placed at the head of the series) fixeson its starting 200 grams, 30 to 35 grams of hvdrate extracted from theliquid phase of the suspension.

In order to maintain in suspension a starting quantity of alumina whichis higher than the precipitable quantity it is, therefore, not possibleto cause this suspension'itself to flow from one decomposer into asucceeding one and so on to the discharge. To this end it would benecessary constantly to renew the head, starting with an importantsupply, for example by means of a starting recycling taken oiT at theend of the operation.

But, besides the material complications which such an operationinvolves, it is accompanied, (if the movements of the re-cycled hydrateare eifected in suspension in an exhausted lve), by

in each of the succeedin devices and thus reducing the volumetricefficiency of the whole unit.

According to the invention it is then necessary to make independent ofeach other in each precipitator the movements of the liquid (sodiumaluminate lye coming from the preceding precipitator) and of the solid(starting nuclei and alumina hydrate already formed in the sameprecipitator) and thus to apply the above described general method ofputting solids in suspension.

However, since the most practical method for conveying the alumina is toeffect this in the form of a thickened suspension in the lye whichcarries it, it will then be necessary to discharge the alumina only fromthe last precipitator of the series, in which the supporting lye isnearly completely exhausted. Thus, the alumina formed in the other.devices is collected in the t last device as its own production proceedsin the latter.

In the accompanying'drawings an example of execution of the device inaccordance with the invention is shown which is particularly adapted tothe decomposition of sodium aluminate in the method of Bayer. In saiddrawings:

Figure 1 is a diagrammatical view showing a precipitator in accordancewith the invention.

Figure 2 is a plan view showing the blade wheel in the upper part of thecentral tube.

Figure 3 is a detail of Figure 1 on a larger scale showing theseparation of the lye from the alumina.

Figure 4 is another detail of Figure 1 on a larger scale showing thedrawing off of the hydrate.

Referring to the drawings, the precipitator shown in Figure 1 comprisesa part cylindrical, part conical container I designed so that its ratioof volume/surface makes possible an elimination of heat sufiicient todetermine the reduction of solubility of the alumina in the lye,together with a mean sojourn time of the aluminate molecules which issufiicient, on the one hand, to make it possible for the molecularmodifications in the decomposition (hydrolysis) to occur and, on theother hand. to'fix the alumina, which said molecular modificationsrendered insoluble, on the starting nuclei after a transitory stage ofmetastable oversaturation. This latter operation also depends on thequantity of nuclei in suspension and on the contact surface of saidnuclei with the lye,

In temperate regions (mean surrounding temperature of 15 C.) and whenonly convection and radiation in free air are used to eliminate theheat, the optimum ratios (volume/surface) are comprised between 1 and1.3. Of course they may be determined for those casesin which operationsare carried on under other climatic conditions, or for those cases inwhich the cooling method would be changed.

Placed in the lower part of the room is an emulsifying cock valve 2which is connected to a branch pipe 3 for the supply of compressed air.-

In container l and coaxial with it is a telescopic tube 45 the upperpart 4 of which is stationary, while the lower part 5 is movable insidethe part 4. Secured at the top of this latter part. are a sheet-metalpiece 6 and a blade wheel 1 provided in their center with an opening thediameter of which is slightly larger than that of the tube 4 and theheight of which is adiusted so that one part of the emulsion may bedeflected by the lower face of the sheetmetal piece and of the bladewheel while the other part falls back on their upper face.

The upper part of container I is surrounded by a frusto-conical ring 9which communicates with said container through a series of openingsprovided in three different horizontal planes at H), II and I2. Theplane of openings I2 is above the normal level !3 of the liquid incontainer I.

The frusto-conical ring 9 comprises an overflow M the discharge plane ofwhich is atthe same height as the openings [2. A cylindrical ferrule 15is also located in the ring so that its base is slightly belows'aid'discharge plane.

The alumina. particles arriving at the bottom of container I are broughtto the form of an emulsion by means of compressed air coming through thepipe 3. The quantity of compressed air which is necessary for theproduction of this emulsion, the density of which must be ;lower thanthe mean density of the hydrate-lye suspension in container i, iscalculated proportionally to the quantity of hydrate which decants in aunit of time and which is itself roportioned:

To the cylindrical section of the precipitator;

To the mean speed of fall or" the particles;

To the mean concentration (ratio alumina hydrate/alumina lye) which itis desired to maintain in the whole of the device for reasons ofefficiency and with a view to adjusting the granulometric composition ofthe alumina; and

To the ratio alumina hydrate/alumina lye in the emulsion; this latterratio is limited, moreover, for reasons of a practical order.

The emulsion which forms rises through the telescopic tube 45 to theupper part of the container, and in this manner the diffusion of the airin the remainder of the vat, on the one hand, and, on the other hand,the eddies which would be produced the friction of the rising emulsioncolumn against the remainder of the susension which a slight downwardmovement is imparted, are avoided.

The fact that the tube is a telescopic tube make it possible:

(a) To adjust its optimum position in operation depending on theclogging of the device and the richness of the emulsion which it isdesired to obtain. The richness will be the higher, all other conditionsremaining the same, the narrower the space between the lower part of thetube and the bottom of the container.

This latter adjustment is obtained through a ming it in the decantedsolid mass and to iacilitate the resumption of operation;

To replace said inner part 5, if necessary, without interrupting theoperation of the decomposer.

The emulsion which arrives at the upper part of tube 5 is discharged anddistributed over the whole surface of the container through thesheetmetal piece 8 or wheel 1. It is possible to obtain a veryhomogeneous distribution since the difierence of the ratios(solid/liquid) in samples taken on one and the same horizontal planedoes not exceed The emulsion is then settled into the annular space Swhich it enters through openings H. It there separates, first, into aclear liquid which is discharged through overflow l4,

second, into light particles carrying air bubbles which return to theupper part of the liquid through openings i2, and third, into heavyparticles which return to said liquid through openins's Ill.

Indeed, the hydrostatic equilibrium in the whole of the device may berepresented in the following manner:

One may suppose that the cylindrical part of the decomposer is filledwith a homogeneous suspension containin 200 grams of alumina hydratehaving a density of 2.3 per litre of suspension with a density of 1.2.

Under these conditions the weight of one litre of the suspension isequal to:

lye

nuclei nuclei alumina lye This difference of density generates a currentflowing from openings ll toward openings to in accordance with arrow 56as a result of which the alumina introduced through openings H in theform of a normal suspension is reintroduced at l G in the form of aslightly thickened suspension.

In order that hydrostatic equilibrium may be maintained at openings Ii,level I? being invariable (overflow) the suspension having a density of1.295 which occupies the precipitator proper enters the frusto-conicaldecantation ring 9 through openings l l and forces out the liquid ofdensity 1.200 which is between the planes of openings 1 l and i2 andwhich flows out through the overflow i l in the direction of arrow l8while the alumina. separated by decantation returns to the decomposerthrough openings Hi.

This process comes to a stop only when level 53 which limits in free airthe height of the suspension the precipitator becomes such that h1.295=h 1.200, h and it being the distances from openings H to thelevels of the liquids in ring :2 and in decomposer l respectively.

It resumed as soon as the precipitator receives a quantity of lye whichtends to break said equilibrium.

In normal operation, there is-therefore always between precipitator iand settling ring 9 a level difference h-h' which is proportioned to thefixed height h and to the richness in solid of the hydrate suspension inthe main body.

This level difference creates a slight current of lye which flows in thedirection of arrow l9 and returns to vessel 1 through openings l2, thefloating alumina which, otherwise, would form skins on the surface infree air of the frusto-conical ring, which would interfere with settlingand cause clogging and choking.

This floating alumina is formed of hydrate grains having provisionallyfixedsmall air bubbles which carry them to the surface. Stopped by theferrule l 5 which opposes its horizontal movement toward the outside itcannot be discharged through overflow l4.

Only the liquid which is perfectly free from any alumina in suspensionis discharged through overflow l4.

It is known that in the Bayer decomposition the solid (in the presentcase the alumina hy drate produced in a decomposer and a certainquantity of starting alumina remaining in the latter) is led to thesucceeding precipitator in the form of a suspension of crystals in thelye.

Now, there is advantage in conveyin the alumina in a state of suspensionwhich is as dense as possible (that is to say with the highest ratiosolid/liquid). For one and the same quantity of hydrate, the volume tobe conveyed is the smaller, the thicker the suspension. This feature isof a particular interest in relation to the final extraction intendedfor feeding the rotary filters, for the use of a thick suspensionpromotes the formation of the layer on the filter.

Moreover, the independence of the movements of the lye and of thehydrate in accordance with the invention makes it possible to maintain,at will, a smaller or larger weight of hydrate in suspension in thewhole precipitator unit, and thus constitutes an excellent stocking anddestocking means. This is all the more so because it is possible eitherto distribute the starting alumina evenly in each of the vats or toconcentrate it in one vat or in another.

During a destocking period, for example, it is possible, in order to beable to dispose of a thick suspension at the feeding of the filters, toconcentrate a large part of the hydrate in the vat in which the finalextraction is effected.

In, drawing off the alumina where the ratio (solid/liquid) is thehighest we have found that the most suitable way consists in effectingthis drawing off in the neighbourhood of the top of the rising column ofemulsion (air-solid-liquid).

This mode of proceeding has the further advantage that since theincrease in concentration in the emulsion is more particularlysubstantial for the largest crystals, and since the drawing off ofhydrate is effected on the rising column at each stage of decomposition,there results a progressive selection which accelerates the extractionof the most developed crystals and leaves the others time to increase.

The difference between the concentration of the rising emulsion and themean concentration may be chosen by varying the quantity of lyedelivered into the emulsion through an adjustment of the position of thelower part of the telescopic tube with respect to the stationary wall ofthe vat.

iA very simple and very practical device for drawing off the suspensionof alumina at will is shown in Figure 4.

This device comprises a tube 29 for the evacuation of the suspension tobe drawn ofi, which tube passes through the wall of the precipitator lat a few decimeters below the discharging plane 11. Inside saidprecipitator and near this wall the tube is swingably mounted, forexample .by means of a rubber sleeve 2 I, permitting a swinging movementof 5 to in the vertical plane.

The free end of this tube comprises a grating 22 which prevents theintroduction of crusts or foreign bodies, and a sheet-metal ring 23, thediameter of which is slightly larger than that of the axial tube 4.

At rest, this end of the tube and the ring 23 are maintained by means ofa rope 24 out of contact with the liquid mushroom formed by the emulsioncurrent issuing from the axial tube and spreading over the sheet-metalpiece 6 or the distribution wheel I.

Under these conditions nothing passes through tube and the output of theprecipitator is discharged through overflow It in the state of adecanted lye.

On the contrary, when it is desired to effect an extraction of aluminait is suflicient to lower tube 20 and ring 23 upon the sheet-metal pieceor the distribution wheel. The ring, while interfering with theestablishment of the emulsion mushroom, then leads toward the opening ofthe tube the thick suspension of alumina which, on account of the slope,is discharged simply through ravity.

In order to stop the drawing off, the ring and the tube are again raisedto the position of rest through pivoting around rubber sleeve 2|.

This device which makes, it possible to efiect all handlings in freeair, avoids the use of mechanical elements and of valve fittings a wellas of pumps and even of sluice-gates which can always be clogged orchoked.

These advantages are more particularly important for suspensions, in anincompletely decomposed lye, of crystallized alumina hydrate the grainsof which have a diameter which is larger than millimeters for, underthese conditions, the least interruption of movement or the least deadspace cause the heaping up of decanted masses, the grains of which sticktogether in a very short time.

From the above description it may be seen that the general method forputting a dense solid in suspension in a liquid in accordance with theinvention makes it possible:

(a) To obtain a very important saving (of more than 50%) of the powerwhich would be necessary to obtain the same result with a mechanicalstirring device;

(b) To maintain in suspension a large quantity of a solid of anygranulometric composition and to distribute it in a homogeneous mannerin all the liquid volume of the device;

(0) To achieve a continuous operation since the movements of the solidand the liquid are completely independent of each other;

(d) To obtain the progressive selection of the grains;

'(e) To attain these results through particularly simple means whichcomprise no mechanical element in contact with products to be treated(doing away with wear and maintenance).

Furthermore, the application of this method to the decomposition througha continuous operation, of the sodium aluminate lyes according to themethod of Bayer, makes it possible besides the saving of power andmaintenance as mentioned above:

(a) To effect savings in volume of the device, and of lye heldstationary, reaching 30 to 35%;

(b) To control the granulometry of the obtained product and to use finenuclei prepared separately, the renewal of the nuclei being no longerleft to the chance of self-precipitation;

(c) To obtain hydrate grains with a radiating structure which makes itpossible to increase the efficiency of the calcining ovens;

(d) To save a considerable part of the initial expenses by doing awaywith the mechanical elements or pumps used with previously knownmethods.

What I claim is:

1. A method of precipitating solid particles from a suspension of theparticles in a liquor, which comprises establishing a column of such asuspension-containing liquor in an open-top vessel, from which theparticles are to be precipitated, and in which is contained a quantityof precipitated particles, providing a centrally located air liftextending from adjacent the bottom of the column to the top thereof,injecting air under pressure into the bottom of the air lift andwithdrawing liquor containing precipitated particles from adjacent thebottom of the column and moving the said liquor upwardly through the airlift to the top of the column, overflowing the liquor moved upwardlythrough the air lift and directing the overflow outwardly in the vesseltoward a decanting zone surroundmg the upper portion of the said columnof liquor, said decanting zone having a downwardly inclined lowersurface sloping inwardly to the said column, said decanting zone beingsubstanat the bottom of said decanting zone, and a thirdlevelintermediate the said first and second levels, introducing the saidoverfiowed liquorinto the decanting zone through the intermediaLe' levelof perforations in the bafiie, and withdrawing a relatively clear liquoras overflow from the periphery of the upper surface of the liquor in thedecanting zone, by concurrently precipitating particles from the liquorin the decanting zone and returning the precipitated particles to thesaid column through the lowermost level of perforation in the bafile andreturning liquor con-' taining relatively fine precipitated particles sur-'- rounded by air bubbles to the said column of liquor through theuppermost level of perforations in the bafiie, the level of the saiduppermost level of perforations in the baffle and the level of theoverflow periphery of the decanting-zone being the same and chosenrelative to the level 'of the overfiowed liquoradjacent the airflift soas to provide fora relatively clear overflow from the periphery of thedecanting zone.

2. In a method according to claim 1 the step 12 of adding freshcondensation nuclei to the liquor in the column.

3. A method according to claimL in which the condensation nuclei arehydrated alumina and the liquor in the column is a solution of sodiumaluminate.

4. A method according to claim 3 in which the proportion of condensationnuclei introduced and maintained in suspension is at least 200 grams perliter of suspension liquor. A SABLE.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Re. 13,668 Fickes Jan. 6, 1914 v 994,679 Hills June 6, 19111,076,666 Dorr Oct. 28, 1913 1,351,155 Brown Aug. 31, 1920 1,434,232Sherwood Oct. 31, 1922 1,732,893 Hunt Oct. 22, 1929 1,847,659 MaclennanMar. 1, 1932 1,952,727 Ralston Mar. 27, 1934 2,385,128 Castner Sept. 18,1945 2,422,499 Pierce June 17, 1947 2,436,509 Haust Feb. 24, 19482,438,204 Castner Mar. 23, 1948 2,460,987 Kanhofer Feb. 8., 1949

1. A METHOD OF PRECIPITATING SOLID PARTICLES FROM A SUSPENSION OF THEPARTICLES IN A LIQUOR, WHICH COMPRISES ESTABLISHING A COLUMN OF SUCH ASUSPENSION-CONTAINING LIQUOR IN AN OPEN-TOP VESSEL, FROM WHICH THEPARTICLES ARE TO BE PRECIPITATED, AND IN WHICH IS CONTAINED A QUANTITYOF PRECIPITATED PARTICLES, PROVIDING A CENTRALLY LOCATED AIR LIFTEXTENDING FROM ADJACENT THE BOTTOM OF THE COLUMN TO THE TOP THEREOF,INJECTING AIR UNDER PRESSURE INTO THE BOTTOM OF THE AIR LIFT ANDWITHDRAWING LIQUOR CONTAINING PRECIPITATED PARTICLES FROM ADJACENT THEBOTTOM OF THE COLUMN AND MOVING THE SAID LIQUOR UPWARDLY THROUGH THE AIRLIFT TO THE TOP OF THE COLUMN, OVERFLOWING THE LIQUOR MOVED UPWARDLYTHROUGH THE AIR LIFT AND DIRECTING THE OVERFLOW OUTWARDLY IN THE VESSELTOWARD A DECANTING ZONE SURROUNDING THE UPPER PORTION OF THE SAID COLUMNOF LIQUOR, SAID DECANTING ZONE HAVING A DOWNWARDLY INCLINED LOWERSURFACE SLOPING INWARDLY TO THE SAID COLUMN, SAID DECANTING ZONE BEINGSUBSTANTIALLY ISOLATED FROM THE SAID COLUMN OF LIQUOR BY MEANS OF AVERTICALLY DISPOSED CONTINUOUS ANNULAR BAFFLE WHICH CIRCUMSCRIBES THEAIR LIFT AT SUBSTANTIALLY THE PERIMETER OF THE SAID COLUMN AND ISPERFORATED ONLY AT THREE DIFFERENT HORIZONTAL LEVELS-ONE LEVEL ABOVE THELEVEL OF THE OVERFLOWED LIQUOR ADJAECNT THE AIR LIFT, A SECOND LEVEL ATTHE BOTTOM OF SAID DECANTING ZONE, AND A THIRD LEVEL INTERMEDIATE THESAID FIRST AND SECOND LEVELS, INTRODUCING THE SAID OVERFLOWED LIQUORINTO THE DECANTING ZONE THROUGH THE INTERMEDIATE LEVELS OF PERFORATIONSIN THE BAFFLE, AND WITHDRAWING A RELATIVELY CLEAR LIQUOR AS OVERFLOWFROM THE PERIPHERY OF THE UPPER SURFACE OF THE LIQUOR IN THE DECANTINGZONE, BY CONCURRENTLY PRECIPITATING PARTICLES FROM THE LIQUOR IN THEDECANTING ZONE AND RETURNING THE PRECIPITATED PARTICLES TO THE SAIDCOLUMN THROUGH THE LOWERMOST LEVEL OF PERFORATION IN THE BAFFLE ANDRETURNING LIQUOR CONTAINING RELATIVELY FINE PRECIPITATED PARTICLESSURROUNDED BY AIR BUBBLES TO THE SAID COLUMN OF LIQUOR THROUGH THEUPPERMOST LEVEL OF PERFORATIONS IN THE BAFFLE, THE LEVEL OF THE SAIDUPPERMOST LEVEL OF PERFORATIONS IN THE BAFFLE AND THE LEVEL OF THEOVERFLOW PERIPHERY OF THE DECANTING ZONE BEING THE SAME AND CHOSENRELATIVE TO THE LEVEL OF THE OVERFLOWED LIQUOR ADJACENT THE AIR LIFT SOAS TO PROVIDE FOR A RELATIVELY CLEAR OVERFLOW FROM THE PERIPHERY OF THEDECANTING ZONE.